This invention relates to semiconductor laser elements and more particularly to semiconductor laser elements of the kind having steps formed by etching away edge portions of its contact layer for an electrode at the top so as to provide regions in the active layer through which currents will not pass when a voltage difference is applied between the electrodes on the top and the bottom.
FIGS. 6A and 6B show an example of prior art method of making such a semiconductor laser element 1, disclosed in Japanese Patent Publication Tokkai 7-30190 laid open on Jan. 31, 1995. This semiconductor element 1 includes a semiconductor substrate 2 on which a lower clad layer 3a, an active layer 4, a first upper clad layer 3b, a current blocking layer 5, evaporation preventing layer 6, a second upper clad layer 3c and a contact layer 7 are sequentially formed in this order one on top of another, and a lower electrode 8a and an upper electrode 8b are formed respectively on the lower surface of the semiconductor substrate 2 and the upper surface of the contact layer 7. The current blocking layer 5 has a stripe-shaped groove 5a formed at its center, and the second upper clad layer 3c is formed so as to fill this groove 5a and to be thereby connected with the first upper clad layer 3b. Steps 9 are provided by removing two edge portions of the contact layer 7.
It is desirable to make the height of the steps 9 equal to the thickness of the contact layer 7 such that the regions not passing current therethrough can be formed in the active layer 4 without adversely affecting the ability of the second upper clad layer 3c to seal in light. Since the etching of the contact layer 7 is controlled by time according to the prior art technology, however, the height of the steps 9 tended to vary, depending on variations in the concentration of the etching liquid and the etching temperature, causing the laser characteristics to become unstable. In other words, if the steps 9 are too high, the second upper clad layer 3c becomes too thin and its ability to seal in light is adversely affected while, if the steps 9 are too low, the contact layer 7 is left in the steps 9, giving rise to the possibility of ill effects on the laser characteristics due to localized heat emission from the active layer 4 as a current is passed through.
It is therefore an object of this invention to provide semiconductor laser elements with reduced variations in the height of their steps such that their laser characteristics can be stabilized.
Another object of this invention is to provide a method of producing such semiconductor laser elements.
A semiconductor laser element according to this invention, with which the above and other objects can be accomplished, may be produced by forming a lower clad layer, an active layer and an upper clad layer sequentially one on top of another on a semiconductor substrate, forming an etching stop layer completely over the upper clad layer, forming a contact layer on the etching stop layer, and etching portions of the contact layer to form steps. The etching stop layer has a slower etching speed than the contact layer against the etching liquid used for forming the steps by etching portions of the contact layer. Thus, the time for the etching process can be set sufficiently long such that the portions of the contact layer can be completely removed and the height of the steps will be uniform and equal to the thickness of the contact layer. An upper electrode is formed on the contact layer and a lower electrode is formed on the bottom surface of the substrate. The steps provide areas in the active layer where a current will not flow through.